1. Field of the Invention
The present invention relates generally to nesting structures for wild birds, and more particularly to an artificial multiple nesting structure simulating the vertical earthen banks, excavations and piles into which certain burrowing birds dig their horizontal nesting tunnels and chambers.
2. Description of the Prior Art
Bank swallows, rough winged swallows and kingfishers once nested only in natural settings such as in the substantially vertical earthen banks of rivers, lakes, oceans, or eroded earthen cliffs. In such settings, they burrow nearly horizontally into the exposed vertical face of such a bank or cliff, creating relatively long tunnels leading to enlarged nest chambers. These birds have also adapted to nesting in semi-natural, man-made, substantially vertical earthen banks, such as might occur in sand pits, clay pits and similar excavations. They will also nest in piles of sand or topsoil where excavating or earth-moving equipment has created vertical faces. Some of these species may nest, rarely or routinely as the case may be, in horizontal drainage pipes set in vertical masonry retaining walls or bridge foundations.
When bank swallows nest in natural sand or clay river banks, etc., they usually choose a high section of bank or cliff face which is nearly vertical. The height and verticality protect the chosen nest sites from climbing predators. Bank swallows prefer bank or cliff faces which offer fine soil relatively free of stones or obstructions to digging, compacted soil supportive of their arched tunnels and nest chambers, and sufficient surface area to accommodate the many closely spaced nests of a colony of numerous breeding pairs. Often these multiple nest entrances are arranged along horizontal lines in the exposed bank face, which correspond with horizontal layers of preferred sand, clay or other soils, as deposited long ago when the bank was formed naturally.
Nest entrances appear as holes in the bank face. Often these entrances are within 12 inches of other entrances. Tunnels approximately the same size as the entrances are dug into the bank approximately perpendicular to the vertical bank face. Tunnels generally slope slightly upward away from the entrance. Tunnels may vary in length, from several inches to a few feet. At the inner ends of the tunnels are enlarged nest chambers, also dug in the earth. Nest entrances, tunnels and chambers are generally (horizontally) elliptical in cross-section or arched with flattened floors.
When digging is finished, bank swallows add straw, grass and feathers to complete a nest in the chamber.
When bank swallows nest in semi-natural man-made situations, such as landfill excavations and loam piles, they usually choose the bank face which is most vertical, most recently cut by the bucket loader, and thus, unfortunately, most likely to be disturbed again soon.
When rough-winged swallows nest in natural or semi-natural settings, it is usually in the earthen burrows created by bank swallows.
When rough-winged swallows nest in man-made drainage pipes set in riverside retaining walls or bridge foundations, these pipes are usually dangerously close to normal water levels, and often below high water marks of flood stage.
When kingfishers nest in natural or semi-natural settings, they are often attracted to the same conditions which attract bank swallows. However, unlike bank swallows, kingfishers do not nest with other pairs of their own kind. Kingfishers, being larger, make and require larger nesting cavities.
Earth-burrowing birds are declining or are underpopulated in some regions. The replacement of many suitable natural river bank nest sites with human development has limited their breeding. The frequent destruction of nest sites in semi-natural, man-made sand pits, landfills, and excavations by ongoing, necessary earth-moving operations, also takes a toll. And the flooding of low-lying drainage pipes during periods of heavy spring rain destroys the nests of birds which resort to such inferior, artificial nest sites.
Beyond the limitations associated with these prior art nesting situations, earth-burrowing birds are entirely prevented from breeding wherever the natural or man-made landscape offers not even an inferior nesting site. Thus, these beautiful and beneficial birds are absent from areas which could otherwise support thriving populations.
The soils in banks, cliffs, piles and excavations may cause problems for burrowing birds. Topsoil or gravel may contain so many stones or roots as to force birds to abandon a site after days of wasted digging. Sand may be so coarse and granular as to collapse on eggs, nestlings or adults.
Heavy rain may cause collapse of nesting tunnels and chambers, or may cause the vertical face of the bank or pile to shear off entirely. Rain may so saturate the soil that eggs, nestlings and adults are dampened and fatally chilled.
Soils which are soft enough to permit burrowing are also soft enough to permit erosion. While erosion, such as the undercutting action of the flow of a river, may create and renew the vertical face of a river bank, collapse of a bank face during the nesting season is disastrous for these birds.
Predation at nesting sites is sometimes a problem for burrowing birds. Although the loose soil, verticality and height of a nesting bank face are resistant to climbing by predators, the relatively soft soil of a nesting bank permits digging by predators, such as foxes, ferrets and badgers.
Native burrowing birds are sometimes attacked or driven from their burrows by destructive alien birds, such as house sparrows. It is difficult to trap sparrows or remove sparrows nests in earthen burrows.
In prior art settings, nests are inaccessible to humans without great effort and great risk of damage and disruption to nestings. Since nest chambers are buried in relatively loose and thus collapsible soil, painstaking and time-consuming care would be necessary to temporarily unearth a single nest chamber for inspection, wildlife management or research purposes. Restoring the nest chamber to permit the nesting to continue would be similarly challenging. The random lengths of nesting tunnels would make estimating the exact location of the nesting chambers difficult. Any exploratory digging by humans from above a sand bank or from behind a loam pile, to access nests, would both weaken the earthen nest site as well as frighten the birds.
In many settings, no digging by humans would be possible without damaging the natural or man-made landscape.
In some prior art artificial settings, nests in drainage pipes set in bridge foundations are completely inaccessible from above or behind.
Burrowing birds may abandon prior art earthen nest sites when these sites become infested with bird lice, mites and the dirty debris from previous nestings. It is difficult, if not impossible, to adequately clean such prior art sites to encourage reuse.
With erosion, collapse and parasite infestation of prior art earthen banks, nesting sites are often not used repeatedly enough to justify the intentional creation of earthen banks by humans to benefit these birds.
If someone did want to create a vertical nesting bank either by cutting away a hillside or dumping and shaping a pile of topsoil, the initial expense and maintenance would be considerable. Heavy equipment, or many, many man-hours would be required to maintain the bank for perhaps just one season's usage.
It further may be illegal to create such a natural nesting site of loose, erodable soil near the protected wetlands these burrowing birds prefer. It may be impossible to transport the large volume and weight of earth necessary for such a created site, through sensitive landscapes.
Finally, prior art multiple compartment birdhouses such as pole-mounted purple martin houses do not ever attract nest burrowing birds. The artificial materials, inappropriate interior dimensions, small external surface areas and elevated isolation from the earth may cause burrowing birds to see no attractive similarity with the earthen banks they nest in.